Field of the Invention
The invention relates to an optoelectronic semiconductor component having a semiconductor body which is suitable for generating electromagnetic radiation. The semiconductor body has an active layer sequence which is disposed on a semiconductor substrate, in particular an n-conducting semiconductor substrate. The electromagnetic radiation is generated in the active layer sequence when a current flows through the semiconductor body. The active layer sequence is associated with at least one poorly dopable semiconductor layer of a first conductivity type, in particular a p-doped II-VI semiconductor layer including ZnSe, a p-doped III-V semiconductor layer including GaN, or an n-doped II-VI semiconductor layer including CdTe.
An optoelectronic semiconductor component of this type is disclosed for example in U.S. Pat. No. 5,268,918, which describes a semiconductor body of a semiconductor laser component, in which an n-conducting or p-conducting first cladding layer, composed of ZnMgSSe or BeZnSTe, is deposited on a semiconductor substrate, composed of n-conducting or p-conducting GaAs or GaP. On the first cladding layer is disposed, in turn, an active layer, composed of ZnSSe or ZnCdSe. A p-conducting or n-conducting second cladding layer, composed of ZnMgSSe or BeZnSTe is provided on the active layer. The optoelectronic component disclosed in U.S. Pat. No. 5,268,918 is based on the principle of the so-called "Separate Confinement Heterostructure" (SCH). In the case of this layer structure, the electrical charge carriers and the light waves generated are guided independently of one another. Electrons and holes are injected into the light-emitting active zone which is configured as a single or multiple quantum well structure (Single Quantum Well, SQW, or Multiple Quantum Well, MQW) and thus has a smaller band gap than barrier layers adjoining the active zone. The barrier layers (waveguide layers) have a smaller refractive index than the active zone and, at the same time, a larger refractive index than the cladding layers adjoining the barrier layers. As result of this difference in refractive index, the light wave generated is restricted to the narrow region around the active zone. By oppositely doping the two cladding layers, a pn junction is produced into which charge carriers are injected when a suitable voltage is applied.
Similar optoelectronic semiconductor components are described for example in the article "II-VI Blue-Green Laser Diodes", by A. Ishibashi, IEEE Journal of Selected Topics in Quantum Electronics, 1, pp. 741 to 748, (1995) and in JP 07-066494.
The fundamental structures of SQW and MQW semiconductor lasers and of the SCH construction are described for example in the book "Halbleiter-Optoelektronik" [Semiconductor Optoelectronics], by W. Buldau, Hanser Publishers, Munich, Vienna, 1995, pp. 182-187, and are therefore not explained in any specific detail at this point.
A problem that frequently arises in semiconductor technology is that certain semiconductor materials are poorly p- or n-dopable. Poorly dopable are in particular II-IV semiconductor materials having ZnSe, (poorly p-dopable; in particular ZnMgSSe or BeMgZnSe) or CdTe (poorly n-dopable), or III-V semiconductor materials having GaN (poorly p-dopable). U.S. Pat. No. 5,338,944 describes for instance a blue light emitting SiC diode in which the problem of poor transparency of p-doped SiC is solved through the use of a degenerated pn junction on a thin p-doped SiC layer.
Particularly in the case of SQW and MQW semiconductor lasers with an SCH construction based on II-VI semiconductor material, in which a p-conducting covering layer (waveguide layer or cladding layer) made of ZnSe-based semiconductor material, in particular ZnMgSSe or BeMgZnSe, is used, this problem impairs the functional properties of the laser to a high degree.
The band gap and also the refractive index of the above-mentioned materials can be set by varying the Mg concentration and the S concentration or the Be and Mg concentrations. With increasing proportions of magnesium and sulfur or beryllium and magnesium, however, the efficiency of the plasma-activated nitrogen which is used as an acceptor for p-type conductivity in the above-mentioned compositions decreases. As a result, layers having relatively high Mg and S concentrations or relatively high Be and Mg concentrations always have a high electrical resistance. Moreover, the resistances of the electrical contacts to these layers are thus increased, as a result of which the properties of the laser component are impaired even further. In conventional II-VI semiconductor lasers, these problems mean that only covering layers with a maximum band gap of approximately 2.95 eV (300 K) are used.
As a consequence of this restriction, the height of the energy barrier between the semiconductor material of the quantum well structure (SQW or MQW) and the cladding layers is considerably limited, as a result of which electrical charge carriers are contained only insufficiently in the quantum well.
However, creating relatively high barriers is important in order to obtain emission wavelengths of less than approximately 500 nm, and so far it has been difficult to achieve high barriers in ZnMgSSe--SCH semiconductor lasers. Furthermore, with low Mg and S concentrations in the cladding layers, the change in the refractive index between the waveguide layers and the cladding layers is relatively small, with the result that a light wave generated in the active zone is guided only weakly.
A further problem arising with known II-VI semiconductor layers is that of forming an ohmic contact to p-conducting layers. In order to improve this ohmic contact on the p-conducting side of the semiconductor laser, a superlattice including ZnSe and ZnTe has generally been used to date. The structural quality of this superlattice is however very poor on account of the considerable lattice mismatch between ZnSe and ZnTe. In addition, BeZnTeSe grading or BeTe/ZnSe pseudograding have also been proposed as p-type contact. In the case of these contact layers, it is technologically difficult to prevent oxidation of the hygroscopic material BeTe at the semiconductor surface. If a BeZnTeSe grading or a BeTe/ZnSe pseudograding is used as an electrically active buffer layer between the p-conducting cladding layer and the semiconductor substrate, p-conducting GaAs is used as the semiconductor substrate since the valence band discontinuity between BeTe and GaAs is small. What is disadvantageous here is that p-conducting GaAs is commercially available only with a quality that is lower than that of n-conducting GaAs. As as a result, the structural properties of the applied epitaxial layers are considerably reduced.